Casting: High or Low Pressure?

[Bloodhammer]

Hi,

Im sure someone will be able to help me with this. Ive spent a long time sculpting my own detailed marine parts and have made silicone moulds of the parts but im getting bubbles due to the detail of the mould. I already own an air compressor and was wondering if connecting it to a pressure pot and placing my moulds within would improve casts.

 

Ive heard that the increased pressure (around 50 psi) crushes the air bubbles within the mould. Is this the case? Has anyone tried this? Ive also heard of vacuum chambers being used to remove bubbles from silicone when making the original mould but this operates on completely the opposite principal.

 

I am confused as to which is correct as they both sound plausible... if they do in fact both work, then which is better?

Thanks for your help.

[GumbaFish]

I know some people have some sort of vibrating platform they place the moulds on while casting that are supposed to encourage the bubbles to rise to the surface instead of getting trapped in the parts you want to cast.

[Bloodhammer]

Ive seen the vibrating tables you mention and whilst they do dislodge bubbles they are not as effective at casting thin elements like cables. Seeing as I already have a compressor it makes sense to get a pressure tank, if it will work of course :D

[GumbaFish]

I thought I'd mention it, I don't have any experience casting but I am fairly certain that those pressure vessels are rather pricey. I know I have seen some videos on youtube about people using them for casting resin though.

[Durus]

This might answer a few of your questions. Link.

The pressure pot he uses with the compressor is normally used to get bubbles out of paint, if I remember correctly, so this could be a budget solution.

 

As for your second question, another way to make sure you don't get bubbles in your molds is to slowly pour the silicone from a certain height into one corner of the mold so that it enters the form in a thin stream and evenly covers your prototype. Though I've never done it myself, because I'm sure I would just mess up the floor of my working space. :)

 

I just use an old brush to evenly cover my originals and make sure that I got every detail, before I pour the rest of the silicone in. Works for casting too, if you use resin. But you have to keep the curing time for both silicone and resin in mind when doing this.

 

And I second the motion to look for instructional videos either on youtube or on the siliconemaker's homepage. There are some good ones out there.

[CarbonCopy]

I have to second Durus suggestion of "painting" a thin layer of RTV silicone to coat the part when making the mold. I use a "flux brush" -- a $0.05 brush you can get at your local hardware store. Do it QUICKLY, just enough to coat the part.

 

When actually casting the part (with resin), "inject" the resin with a basting dropper (usually called a "turkey baster").

[Jakael]

To use a pressure pot your molds need to made in a vacuum. this removes the bubbles from the mold. if you cast in a pressure pot the tiny bubbles in your rubber will pop and wreck your mold and cast. Other than that you are correct, it would improve your casts. 1 thing i tried recently was painting the two halves of my mold with epoxy glue and then putting the two halves together to cure. I got zero air bubbles. there was a lot of flash but that is too easy to remove. give it a try, if you use clear epoxy glue you can see any air bubbles that may form and pop them with a toothpick before you put the 2 mold halves together.

[chewielight]

There are a few reasons bubbles will come into molds when you are casting.

They are:

1. Mold is being poured too fast.

2. High detailed molds.

3. Cold molds.

4. Not enough release agent (ie. Spray,baby powder etc.

5. contaminent (water usually the issue and this will cause foaming)

 

The best ways to combat some of these are as follows:

1. slow down and pour from the edge until it is filled. Dont move the pour around it will cause air to get in the cast.

2. Major cause of bubbles here pour a small amount resin and swish around until you coat all of the mold then pour slowly more resin to help fix bubbles from uncut or high detailed castings.

3. Cold molds can cause bubbles from the heat reaction inside while resin is curing. I heat my molds in the microwave for about a minute and have not had any problems in a long time.

4. I use both I will normally coat the mold in mold release and give it time to completely dry. My wife works for a talc company so I just use the finest powder I can get but baby powder works just as well, and dust the mold. Nothing better to keep molds from sticking.

5. If you are getting foaming bubbles that means you had moisture of some type get into either the resin or mold. one thing you can do if you see foaming in your cast is to bake it in your oven at about 400 for about an hour. Make sure your silicone can handle that kind of temp but most can up to 500+ so just read the instructions with your silicone. just leave the resin inside the mold and let the stuff bleed off. You will probably have to sand those parts but it helps keep the mess down.

[chewielight]

@jackael

 

yes using a pressure chamber can suck when making molds. If you use a silicone that bubbles easily then use the vacum before you actually make the mold. Then pour for your mold and do it slowly as to keep the bubbles down from the pour. I have found that using a vaccume chamber right after your mix your silicone keeps everything nice.

never tried to glue mold parts together but what I do is when making two part molds I just usally put small dowl rods into it so that both part line up correctly. Then I demold and when i am ready to cast they plug together just right. I use wood and rubber bands or electrical tape to hold them both together.

 

 

 

Hint:

Also in two part molds cutting vents into certain areas helps keep things like swords and sharp angels from getting bubbles.